Overriding public virtual functions with private functions in C++

Question

Is there is any reason to make the permissions on an overridden C++ virtual function different from the base class? Is there any danger in doing so?

For example:

class base {
    public:
        virtual int foo(double) = 0;
}

class child : public base {
    private:
        virtual int foo(double);
}

The C++ faq says that it is a bad idea, but doesn't say why.

I have seen this idiom in some code and I believe that the author was attempting to make the class final, based on an assumption that it is not possible to override a private member function. However, This article shows an example of overriding private functions. Of course another part of the C++ faq recommends against doing so.

My concrete questions:

1. Is there any technical problem with using a different permission for virtual methods in derived classes vs base class?

2. Is there any legitimate reason to do so?

Answer 1

You do get the surprising result that if you have a child, you can't call foo, but you can cast it to a base and then call foo.

child *c = new child();
c->foo; // compile error (can't access private member)
static_cast<base *>(c)->foo(); // this is fine, but still calls the implementation in child

I suppose you might be able to contrive an example where you don't want a function exposed, except when you are treating it as an instance of the base class. But the very fact that that situation pops up would suggest a bad OO design somewhere along the line that should probably be refactored.